Spherical plant viruses: interactions in solution, phase diagrams and crystallization of brome mosaic virus

Brome mosaic virus (BMV) is a small icosahedral plant virus of mean diamete r 268 Angstrom. Interactions between BMV particles in solution were studied by means of small-angle X-ray scattering in order to rnd crystallization c onditions. The interactions between biomacromolecules as large as these vir uses have not yet been systematically studied by this method. As it is know n that usually proteins crystallize in, or close to, attractive regimes, th e interactions between BMV particles in solution were studied as a function of pH, type of salt and size and concentration of polyethylene glycol. An unexpected result of these studies is that the precipitates obtained upon a ddition of PEG alone or PEG combined with salt were in fact made of microcr ystals, which were all characterized by the same series of diffraction peak s, with positions close to those of a centered cubic space group. A phase d iagram of the virus as a function of PEG concentration was established by m eans of microbatch experiments. From the precipitation zones, conditions fo r crystallization were tested from 5 to 40 mg ml(-1) virus with 3-10%(w/v) PEG 8000 or PEG 20000. Small crystals were obtained in several conditions a fter a few days and continued growing for several weeks.